Over the course of the campaign, guilt-inducing subjects lines weren’t the most successful. That honor went to casual introductions. “Hey,” for example, brought in millions of dollars, according to Businessweek.

The lesson is that a familiar tone beats one that’s more formal.

“The subject lines that worked best were things you might see in your inbox from other people,” Toby Fallsgraff, director of Team Obama’s email staff, told Businessweek.